Misaligned flags dark theme /configui firefox

Description

As reported by slumlord. Reproduced by me on firefox 64.
Also affects first wizard pane.
Problematic CSS is at lines 3488 and 7495, blocks starting with:
@media screen and (-webkit-min-device-pixel-ratio:0)

But removing/disabling those sections causes flags to be misaligned on chrome. Those sections are a standard way to have css that only webkit-based browsers (e.g. chrome) will recognize, but firefox as of ff 49 started recognizing the webkit CSS identifiers, so it does see them. But firefox still doesn't behave the same as chrome, somehow. The light theme doesn't have those sections, or this problem, but it isn't clear how to make dark work like light does. Also problematic are the different rules for whether the flag is checked or not, and for different screen sizes.

No changes to dark css since 0.9.37-0, and Alex reports it was an issue in that release.

Alex said he couldn't work on this due to various dependencies with unknown completion dates. So I tried to fix it for several hours but this is as far as I could get, I'm stuck and just thrashing at this point, wasting too much time, so I'm stopping. If it doesn't get fixed for 38, it's not great, but it's the non-default theme, and if you're running the wizard on first install you'll be on light.

Alex thought he had a fix at the last minute but when I tested it, it worked on firefox but broke chrome, same as my report in OP. So I didn't include it in 38. But the patch was mangled by the pastebin so that may have been it. Or not. Let's get this in 39, one way or another.